1. Field of the Invention
This invention relates to a chip breaking device for annular cutters with an improved chip-removing and cutting efficiency.
2. Description of the Related Art
Although a twist drill having two cutting edges can be used for a boring operation, it is more usual for an annular cutter to be employed, because it has a small driving motor and shortens the cutting time. This annular cutter has a rotatable hollow-cylindrical body provided with a plurality of cutting edges arranged in circular fashion on its lower end edge. The annular cutter cuts a narrow annular groove by means of the cutting edges, thereby to effectively form a hole where one is required.
Chips cut by the cutting edges of the annular cutter are removed from the annular groove being formed to the outside of the annular cutter via the flutes which are formed in the outer periphery of the body and are adjacent to the corresponding cutting edges. Chips being continuously pushed out of the flutes tend easily to wrap around the annular cutter, and as the annular groove is cut deeper, so the amount of the chips wound around the annular cutter becomes greater, easily clogging the flutes. This lowers the chip-removing efficiency and increases the cutting resistance. Finally, such clogging may ultimately lead to the boring operation having to be stopped. If this occurs, the stopped cutter must then be removed from the work and the chips manually removed. This troublesome and time-consuming operation inevitably leads to a lower cutting efficiency.
In order to overcome this problem, there has been proposed a chip breaker for annular cutters having a lug extending parallel with the outer periphery of the annular cutter, as described in the specification of U.S. Pat. No. 4,514,120. The lug is arranged close to the outer periphery of the annular cutter. The chips being ejected from the flutes abut against the lower end portion of the lug and are forcibly bent. Thereafter, they are scattered in the centrifugal direction.
Chips resulting from the formation of an annular groove in a work may curl outside of the flutes, may flail about irregular fashion, due to changes in the cutting conditions, and may vary slightly in their respective thickness.
When chips abut against the lug of a conventional chip breaker, they are often not broken and instead are moved upward along the cutter, while remaining wrapped therearound. Once they begin to wind themselves around the cutter, the amount of the chips wrapped therearound increases rapidly, greatly reducing the chip-removing efficiency. If chips become very tightly wound around the annular cutter, the boring operation must then be interrupted or else the chips must be removed from the cutter before the next boring operation begins. Not only is this a time-consuming process; there is a strong possibility of injury to the operator, through his fingers, for example, being badly cut by a sharp chip.